Spondylolisthesis is a condition in which a bone (vertebra) in the lower part of the spine slips out of the proper position onto the bone below it. For example, one vertebra may be properly aligned with the spinal column and another adjacent vertebra may be misaligned and slipped over the aligned vertebra. In children, spondylolisthesis usually occurs between the fifth bone in the lower back (lumbar vertebra) and the first bone in the sacrum (pelvis) area. It is often due to a birth defect in that area of the spine (such as spondylolysis) or sudden injury (acute trauma). In adults, the most common cause is degenerative disease (such as arthritis or spondylosis). The slip usually occurs between the fourth and fifth lumbar vertebrae.
Other causes of spondylolisthesis include bone diseases, traumatic fractures, and stress fractures (commonly seen in gymnasts). Certain sport activities, such as gymnastics, weight lifting, and football, put a great deal of stress on the bones in the lower back. They also require that the athlete constantly overstretch (hyperextend) the spine. This can lead to a stress fracture on one or both sides of the vertebra. A stress fracture can cause a spinal bone to become weak and shift out of place.
One form of treatment for spondylolisthesis involves inserting a system of pedicle screws with rod-receiving polyaxial heads that receive rigid rods to link two or more vertebrae. Typically, the screws may be inserted into the pedicles of two adjacent vertebrae during surgery. Screw extenders may be attached to the polyaxial heads of the screws to facilitate insertion of the surgical rods and provide leverage that increases torque for aligning the spine. The screw extenders may include substantially tubular bodies of a uniform diameter. A surgical rod may then be placed in line with the spine and coupled to one of the screws attached to the aligned vertebra. In order to properly align the slipped vertebra, the surgical rod must be coupled to a screw attached to the slipped vertebra. Typically, an instrument known as a rod reducer may be used to “reduce” the surgical rod into the polyaxial head. Alternatively, the rod reducer may grasp the polyaxial head and “pull” the polyaxial head onto the surgical rod. Once the surgical rod has been coupled to both polyaxial heads, setscrews may be inserted to lock the rod in place.
At times, it may be desirable to also compress or distract the two vertebrae as well in order to maneuver various portions of one vertebra away from the other. Compression or distraction may be necessary to increase or decrease the interdiscal space between the adjacent vertebrae. This may be necessary for various reasons: relief from slipped or herniated discs, spacing for pinched nerves, and/or insertion of supportive interbody spacers and bone grafts for bone fusion. Typically, an instrument known as a compressor-distractor may be used to compress and distract the two vertebrae.
Thus, during the surgical procedure, one instrument must be used to reduce the rod into the screw head to align the vertebrae and another instrument must be sued to compress and/or distract the vertebrae using the screw extenders. Such procedures may require a surgeon to frequently alternate between the various instruments. For example, a compressor-distractor instrument may be coupled to the screw extenders to compress and distract two or more vertebrae. After compression and/or distraction, the surgeon may use a rod reducer instrument to reduce the rod into the screw head until the rod is properly seated. Frequent switching back and forth between these instruments increases surgery times and increases the chance for error on the part of the surgeon. Therefore, it would be desirable to have an instrument that performs multiple functions on the screw extenders and vertebral segments.